1. Field of the Invention
This invention relates to a subsea valve apparatus to control the flow of fluid in a pipe. More particularly, this invention relates to a subsea valve that is insensitive to ambient hydrostatic pressure and automatically closes to stop fluid flow in a pipe when a loss of hydraulic operating pressure occurs.
2. Description of the Prior Art
As the production of oil and gas expands to deeper water depths, the use of fixed offshore structures which extend from the sea floor to above the water surface eventually becomes economically infeasible. One of the more practicable alternatives is the use of a subsea production system (see U.S. Pat. No. 3,777,812). However, in deep water environments it is essential that all the valves on a subsea production system used to control the flow of oil and gas be insensitive to the ambient hydrostatic pressures. That is, the operation of the valve under all conditions should not be affected by the local water pressure. In addition, it is important that these valves be fail-safe. If the hydraulic or pneumatic operating pressure fails, the valve should automatically close the pipe or pipeline to prevent further fluid flow.
Pressure insensitive (sometimes referred to as "balanced"), fail-safe valves which close or shut off the flow of fluid in pipelines during the loss of hydraulic operating pressures or the like are known in the art (see, for example, U.S. Pat. No. Re. 30,115). Such underwater valves generally include a piston-operated valve stem which reciprocates within a cylinder to open and close a gate. In a single acting valve, hydraulic pressure or power fluid forces the piston in one direction, commonly referred to as a power stroke, and a spring means returns the piston to its starting position, commonly referred to as an exhaust stroke. In the event the hydrostatic pressure acting on the exposed end of the piston (the difference between the operating pressure and the valve pressure multiplied by the piston stem diameter) is greater than the spring load in the piston operator, the valve will remain in the open position. If an opposing valve stem capable of contacting the valve element is added as illustrated in Reissue 30,115, the unwanted hydrostatic force exerted against the stem is balanced thereby permitting the spring to close the valve even when the ambient pressure is greater than the internal valve pressure. Reissue 30,115 discloses a balanced stem, fail-safe valve system which includes a balancing stem detached from the main valve element but engagable with it, thereby permitting the stem to balance the valve when the hydrostatic pressure is greater than the internal pressure. However, since the stem is detached, the valve operates as a conventional fail-safe valve when the ambient hydrostatic pressure is less than the internal operating pressure.
While fail-safe, pressure insensitive valves such as that disclosed in Reissue 30,115 are available, a need exists for a remotely operable, fail-safe, pressure insensitive valve which can be quickly attached and detached from a pipeline with a subsea manipulator tool or the like. This need is accentuated as oil and gas is produced in deeper water depths requiring the use of subsea valves which are not readily installed, maintained or removed by conventional diving techniques.